In most of the cases you pointed out for multiple domains having the same MX
host (or A record, or ...) i'd be willing to specify a channel/host pattern to
match it -- thus something like
smtp/*.nzl.xerox.com ...
smtp -e ... $host
I wouldn't expect the scheduler to just automagically detect that anything that
is being scheduled on some SMTP channel might possibly be eligible for one of
the MX checks.
I think that you might want to reconsider creating the DNS cache -- you'll
probably end up causing some trouble because while the DNS servers will get
notification of zone updates (per the latest implementations) your cache won't.
In my situation I can run an authoritative DNS server on the same machine as
zmailer is running on, and that server collects its own large cache --
including the negative cache information (which zmailer doesn't have, and we
could really do with as an option on some of its databases). Rather than put
time into hacking too much more on the ZMailer DNS code, i'd be tempted to
suggest that people run a forwarding DNS server, and rely on its cache.
\nick